In emission computed tomography (ECT), images of internal structures of the body are generated by injecting a patient with a radiopharmaceutical and then using a gamma camera to detect emitted gamma rays, or photons, from various angles around the body. Photons reaching a gamma camera's scintillation detectors produce scintillation events, which are detected by photomultiplier tubes and then converted by circuitry into electrical signals representing both position and energy information. These signals are then used to generate image data in a process known as reconstruction.
An effect known as photon attenuation is an important factor which affects the accuracy of images collected using ECT. Photon attenuation tends to degrade images by introducing image artifacts and other distortions that can result in false positive detection of lesions or the failure to detect lesions. Non-uniform photon attenuation creates image degradation by interfering with and partially absorbing the radiation emitted from an organ containing the radiopharmaceutical. Since each patient that is imaged using a gamma camera is different (different shape, different size, etc.), the tissue and bone structure surrounding an organ of interest are different for each patient. This surrounding tissue and bone structure attenuates the radiation emitted from a radiopharmaceutical distributed within the organ. The attenuation of the radiation is non-uniform because the attenuation coefficients of the different tissues and bone are different. Hence, radiation attenuation non-uniformly reduces the count density in the images of views from different angles. This attenuation can lead to falsely identifying an artifact when, in fact, healthy tissue is imaged and vice-versa.
Non-uniform attenuation caused by the body can be compensated for if an "attenuation map" of the body is determined. An attenuation map comprises a number of attenuation coefficient values corresponding to different points within the body. Transmission computed tomography is a technique which allows a gamma camera and a processing computer system to generate a non-uniform attenuation map of an individual patient. Generally, during transmission scanning, radiation from a transmission source having known properties is transmitted through the patient and then detected by a scintillation detector. By knowing the intensity of the radiation transmitted by the source and measuring the intensity of radiation which passes through the object, a computer within the gamma camera system can measure the extent of non-uniform attenuation over different parts of the body at different angles. From this information, a non-uniform attenuation map of the body can be reconstructed using well-known methods and procedures. The non-uniform attenuation map is then used during the emission reconstruction process to correct emission image data collected during ECT imaging.
Transmission scanning and emission scanning are often performed at different energy levels to allow simultaneous acquisition. Consequently, the attenuation map, which is initially based on the transmission energy level, must be calibrated to the emission energy level in order to use the attenuation map to correct emission image data. For example, a transmission scan might be performed at an energy level of 100 keV using Gd-153, while the emission scan is performed using an energy level of 140 keV using Tc-99 m. Calibration of the attenuation map generally involves scaling the coefficients of the attenuation map to correspond to the emission energy level. Calibration is performed by the computer in the gamma camera system executing computer program instructions that define the scaling operation. The use of an accurate scaling factor in calibrating the attenuation map is necessary for generating accurate emission images. However, for various reasons, the determination of an accurate scaling factor for calibration is problematic. One reason such a determination can be difficult is the scattering within the body of photons emitted from an organ of interest.
Therefore, it is desirable to provide for more accurate calibration of an attenuation map for use in correcting emission image data in ECT.